DE-B-1 126 157 shows a double-beam spectrophotometer with a single chopper, i.e., a circulating beam interrupter. Two light beams originate from a common light source. One of the light beams transmits in a measuring path of rays through a measuring cuvette, the other light beam transmits in a reference path of rays through a reference cuvette. By means of the chopper, the two light beams are superimposed onto a common path of rays, with the measuring and reference light beams being alternatingly guided by the chopper onto a common path of rays. The measuring and reference light beam which simultaneously nest into each other in one path of rays impinge on a common detector. The absorption measurement is made by optical alignment by means of a comb diaphragm in the reference path of rays.
Indeed, only one chopper is present. However, the measuring and reference light beams originate from the light source into different directions, and are therefore from the beginning not identical. In some light sources, e.g., a hollow cathode lamp in the atomic absorption spectral photometry, the generation of two such light beams running in different directions is impossible.
U.S. Pat. No. 3,924,950 shows an atomic absorption spectrophotometer with a measuring light beam and a reference light beam. The measuring light beam passes through a sample area. There, a flame or a furnace is provided as an atomizing device. The reference light beam bypasses the sample area. In this double-beam photometer, a single light beam originates from the light source, a hollow cathode lamp. This light beam is split by a beam splitter into two partial light beams having a fixed intensity ratio. The beam splitter forms a "splitting location". By means of a second beam splitter, or rather "beam combiner", the measuring light beam, which was passed through the sample area, and the reference light beam, are again recombined. The second beam splitter forms a "recombination location". The light beam generated by this superimposition supplies a monochromator and a detector. The monochromator and the detector can together be designated as "detector means". A single chopper alternatingly interrupts the measuring light beam, the reference light beam, or both light beams.
In this arrangement, considerable light losses occur through the beam splitter. The chopper does not have the function to split or to recombine light beams but only effects an alternating interruption.
EP-A2-0 014 357 describes a double-beam changing light colorimeter in which the measuring bean coming from a lighting optic is alternatingly deflected by means of a control organ and stationary mirrors onto a first measuring cuvette and, subsequently, through a second measuring cuvette onto a common photo-element. The control organ is a mirror which can be moved into the path of rays of the measuring beam and which deflects the measuring beam generated by the light source by 90.degree..
Here, the movable mirror causes a changeover between the measuring and the reference path of rays. However, there is no recombination of the measuring and the reference light beam.
EP-B1-0 084 391 shows another arrangement having mirrors which can be moved into the path of rays. There, also, the recombination is made by a movable mirror.
DE-C2-23 03 533 shows an atomic absorption spectrophotometer which is designed as a double-beam apparatus with a measuring path of rays containing an atomized sample substance and a reference path of rays. The apparatus described there is also designed to take into consideration background absorption. The apparatus comprises a line-emitting first light source which emits the resonant line of an element which is to be measured. Further, the apparatus comprises a second light source which emits a continuous spectrum. Optical systems generate light beams which originate from the first and second light sources, respectively, and the beam axes of which intersect each other. A monochromator selects a limited spectral range containing the resonant line from the entire, continuous spectrum. Then, the measuring and reference light beams impinge upon a detector. In a predetermined and cyclic succession of four consecutive intervals, a chopper arrangement passes the light beam from the line-emitting first light source through the measuring and reference path of rays, and the light beam from the light source providing a continuous spectrum through the measuring and reference path of rays onto the monochromator and the detector. There, the chopper arrangement is provided with two chopper discs which are equiaxially spaced in an axial distance and circulate together. Both chopper discs are optically arranged in front of the measuring and reference path of rays in the area of the intersecting points of the beam axes of the light beams originating from the two light sources. Reflecting and transparent surfaces are arranged on the chopper discs so that only one of the light beams, originating from the light sources, is guided to one of the path of rays (measuring or reference path of rays).
DE-A-20 56 112 shows a double-beam photometer with two choppers for separating and recombining the measuring and reference light beams. There, the problem of the synchronization of the two choppers occurs.
DE-A-19 11 048 shows a chopper arrangement with a rotating sector mirror by which the light beam originating from a hollow cathode lamp is passed once into a measuring path of rays and once into a reference path of rays. The sector mirror is also provided with a mirror sector on its side remote from the hollow cathode lamp. A deuterium gas discharge lamp is arranged behind the sector mirror and emits a continuous spectrum. The light originating from this deuterium lamp is reflected by the back of the sector mirror into the reference path of rays into which the light from the hollow cathode lamp transmits from another position of the sector mirror. In a transmission position of the sector mirror, the light from the hollow cathode lamp transmits in the usual way into the reference path of rays, while, in another transmission position of the sector mirror, light transmits from the deuterium gas discharge lamp into the measuring path of rays and the reference path of rays is covered by a covering diaphragm.